<?php

    function ReklamEkle($ayrac, $n=3, $text, $reklam){
        $temp = explode($ayrac, $text);
        $temp[$n] .= $reklam;
        
        return implode(null, $temp);
    }
    
    $text = <<<EOF
    
    <p> 1. paragraf </p>
    <p> 2. paragraf </p>
    <p> 3. paragraf </p>
    <p> 4. paragraf </p>
    <p> 5. paragraf </p>
    <p> 6. paragraf </p>
    <p> 7. paragraf </p>
EOF;


    echo ReklamEkle("<p>", 4, $text, "<p><strong>reklam</strong></p>");